1. Field of the Invention
The present invention relates to a retrofocus wide-angle lens system to be used in a single lens reflex camera.
2. Description of the Related Art
In a single lens reflex camera, in order to maintain a sufficient back focal distance, a retrofocus lens system, in which negative front lens group (the first lens group) and a positive rear lens group (the second lens group) are provided in this order from the object, has been widely used as a wide-angle lens system. In particular, in a conventional retrofocus wide-angle lens system having the half angle of view of about 32xc2x0 to 36xc2x0, if an attempt is made to miniaturize the lens system, the optical performance thereof is deteriorated, so that miniaturization of the lens system is not sufficiently attained.
It is an object of the present invention to provide a miniaturized retrofocus wide-angle lens system in which the half angle of view is about 32xc2x0. Furthermore, it is another object of the present invention to provide a miniaturized wide-angle lens system which has a high optical performance.
In order to achieve the above mentioned objects, there is provided a wide-angle lens system including a negative first lens group, and a positive second lens group on which a diaphragm is mounted, in this order from the object. The wide-angle lens system satisfies the following condition:
xe2x88x921.4 less than f1/f less than xe2x88x921.1xe2x80x83xe2x80x83(1)
wherein
f1 designates the focal length of the first lens group, and
f designates the focal length of the entire lens system.
As defined in condition (1), if the negative power of the first lens group is made stronger, miniaturization can be attained.
In order to correct aberrations occurred in the negative first lens group, the most image-side lens element in the second lens group is preferably a positive meniscus lens element having a convex surface facing toward the image. Further, an aspherical surface is preferably provided on the object-side surface of this meniscus lens element. The aspherical surface is formed so that the radius of curvature becomes smaller than that of the paraxial spherical surface along with an increase of the distance from the optical axis toward the periphery, and thereby positive power of the positive meniscus lens element is weakened; and still further the aspherical surface preferably satisfies the following condition:
xe2x88x925.5 less than xcex94X(nxe2x88x921)xc3x971000/f less than xe2x88x922.0xe2x80x83xe2x80x83(2)
wherein
xcex94X designates the amount asphericity with respect to the paraxial spherical surface at a height of 0.3f from the optical axis; and
n designates the refractive index of a lens material by which the aspherical surface is formed.
It is noted that the aspherical surface can be formed directly on the lens surface, or can be formed as a hybrid lens element in which a plastic layer is provided on a glass lens element.
The second lens group preferably includes a positive 2a sub lens group, the diaphragm, and a positive 2b sub lens group, in this order from the object. The 2a sub lens group includes a cemented sub lens group having a positive lens element and a negative lens element. The 2b sub lens group includes a negative lens element, a positive lens element, and the positive lens element, in this order from the object. Furthermore, the second lens group preferably satisfies the following conditions:
xe2x88x920.8 less than r2a/f less than xe2x88x921.0xe2x80x83xe2x80x83(3)
1.5 less than fL6/fL5 less than 5.0xe2x80x83xe2x80x83(4)
wherein
r2a designates the radius of curvature of the cemented surface of the 2a sub lens group;
fL5 designates the focal length of the object-side positive lens element in the 2b sub lens group; and
fL6 designates the focal length of the image-side positive lens element in the 2b sub lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No.Hei-11-36100 (filed on Feb. 15, 1999) which is expressly incorporated herein by reference in its entirety.